My car jolts up with force harder than usual when switching gears, why?

So I have a Honda civic 1998, just today, I noticed that when I switch from Park to D4, my car jolts because of the shifting gear. It never used to jerk that hard. When I accelerate while driving, my car jerks forward harder than it usually does too. I've already noticed subtle jerks when accelerating, but now it jolts forward kind of hard where my head is kind of pushed backwards when it does this. Can someone please help diagnose the problem?